The present invention relates generally to nonwoven fabrics, and methods for producing such fabrics, and more particularly to a hydroentangled, low basis weight nonwoven fabric exhibiting desirable softness and strength characteristics in a three-dimensional patterned form, with manufacture from a lightly bonded precursor web facilitating efficient and high-speed production.
Nonwoven fabrics are used in a wide variety of applications where the engineered qualities of the fabric can be advantageously employed. These types of fabrics differ from traditional woven or knitted fabrics in that the fibers or filaments of the fabric are integrated into a coherent web without traditional textile processes. Entanglement of the fibrous elements of the fabric provides the fabric with the desired integrity, with the selected entanglement process permitting fabrics to be patterned to achieve desired aesthetics.
Various prior art patents disclose techniques for manufacturing nonwoven fabrics by hydroentangling processes. U.S. Pat. No. 3,485,706, to Evans, hereby incorporated by reference, discloses a hydroentanglement process for manufacture of nonwoven fabrics. Hydroentanglement entails the application of high-pressure water jets to webs of fibers or filaments, whereby the fibers or filaments are rearranged under the influence of water impingement. The web is typically positioned on a foraminous forming surface as it is subjected to impingement by the water jets, whereby the fibers or filaments of the web become entangled, thus creating a fabric with coherency and integrity, while the specific features of the forming surface act to create the desired pattern in the nonwoven fabric. However, there is no teaching or suggestion in Evans ""706 to form a fabric upon a three-dimensional forming surface.
Heretofore, typical hydroentanglement of relatively low basis weight fabrics with the Evans-type technology has been problematic. At low basis weights (on the order of less than 30 grams per square meter), there are a relatively low number of fibers or filaments present for entangling, thus making entanglement relatively inefficient. Entanglement of these light basis weight webs on traditional forming surfaces taught by Evans and its progeny tends to xe2x80x9cwashxe2x80x9d the low fiber content webs, rearranging the fibers in a fashion which undesirably results in a non-uniform product. Entanglement of these low basis weight webs at relatively high processing speeds compounds the problem of maintaining uniformity, because the impinging water jet flows and/or pressures must be relatively increased, which increases the undesirable tendency to distort the web. Further, the high energy jets required by high speed entangling processes tend to drive the fibers into the drain hole openings of the foraminous surface, or into the interstitial spaces of a woven forming wire. This creates serious difficulties with web transfer.
U.S. Pat. No. 5,369,858, to Gilmore et al., discloses a process for forming apertured nonwoven fabric from melt-blown microfibers using the Evans-type technology. These types of fibers are attenuated during known melt-blowing formation techniques, whereby the fibers have relatively small diameters. This patent discloses the use of a belt or drum forming surface having a perforated or foraminated forming surface. Plural hydroentangling manifolds act against fibers positioned on the forming surface to displace the fibers from xe2x80x9cknucklesxe2x80x9d of the forming surface, and into openings or lower parts of the forming surface topography, as in Evans. This patent contemplates use of a polymeric net or scrim for fabric formation, and the formation of fabric having apertures therein of two different sizes, including formation of fabric from a first layer of textile fibers or polymeric filaments, and a second layer of melt-blown microfibers.
U.S. Pat. No. 5,516,572, to Roe, discloses a disposable absorbent article including a liquid pervious topsheet, wherein the topsheet comprises a nonwoven fabric prepared from a homogeneous admixture of melt-blown fibers and staple length synthetic fibers. The patent contemplates that fabrics formed in accordance with its teachings comprise a blend including up to 50% by weight of melt-blown fibers.
U.S. Pat. No. 4,805,275, to Suzuki et al., also discloses a method for forming nonwoven fabrics by hydroentanglement. This patent contemplates that hydroentanglement of a fibrous web be effected on a non-three-dimensional smooth-surfaced water-impermeable endless belt, but notes that at fabric weights below 15 grams per square meter that irregularities in the fibrous web occur, and fabrics with substantial uniformity cannot be obtained.
In contrast to the above-referenced patents, the present invention contemplates a process employing a three-dimensional image transfer device for forming relatively low basis weight nonwoven fabrics, which can be efficiently practiced for manufacture of patterned fabrics having a high degree of uniformity. Such uniformity facilitates use of such fabrics in a wide variety of applications, with efficient formation facilitating economical use.
A process of making a nonwoven fabric having a low basis weight in accordance with the principles of the present invention contemplates hydroentangling on a three-dimensional image transfer device of a precursor web comprising spunbonded continuous polymeric filaments. As is known in the art, spunbonding entails extrusion or xe2x80x9cspinningxe2x80x9d of thermoplastic polymeric material with the resultant filaments cooled and drawn or attenuated as they are collected. The continuous, or essentially endless, filaments may be bonded, with the process of the subject invention contemplating that such spunbonded material be employed as the precursor web.
To form relatively low basis weight fabrics, a precursor web having a basis weight from about 10 to about 30 grams per square meter is employed. The present invention further contemplates that a three-dimensional image transfer device be provided, with the transfer device having a fabric-forming surface defined between three-dimensional surface features. Preferably, at least some of the surface features have profiles which converge toward each other in a direction toward the fabric-forming surface, with the presently preferred image transfer device comprising rectilinear pyramidal array.
With the precursor web positioned on the image transfer device, hydroentanglement is effected by application of high pressure liquid streams to the web. Filaments of the web are rearranged by the fabric-forming surface of the image transfer device, including movement of at least some of the filaments off of the three-dimensional surface features of the device to regions of the forming surface between adjacent ones of the surface features. In the preferred embodiment, wherein a pyramidal array is employed for the image transfer device, filaments are displaced and compacted under the influence of the liquid streams, to regions between adjacent ones of the pyramids of the array. The three-dimensional image transfer device, thus acts in concert with the high pressure liquid streams, to rearrange the filaments of the precursor web relative to the (vertical) Z-axis of the web, as well as relative to the X-axis and Y-axis.
A low basis weight web formed in accordance with the present invention comprises a web of hydroentangled polymeric filaments having a denier from 0.2 to 3.0. The filaments are arranged in a substantially uniform array including interconnected bundles of filaments surrounding apertures extending through the web. The fabric has a basis weight of from about 10 to about 30 grams per square meter, a cross-direction tensile strength of at least about 64 grams per centimeter at 59% cross direction elongation, and a machine direction tensile strength of at least about 242 grams per centimeter at 24% machine-direction elongation.
Notably, the characteristics of the spunbonded precursor web, in particular the strength of its bonds, has a direct influence on the strength characteristics of the resultant low basis weight fabric. Development has shown that if the spunbound precursor web is only relatively lightly bonded, hydroentanglement acts to break or disrupt the bonds without substantially breaking the continuous filaments from which the spunbond precursor web is formed. As a consequence, a low basis weight fabric formed in accordance with the present invention may be formed to include substantially continuous filaments (from a relatively lightly bonded spunbond precursor web), with the resulting fabric having a machine direction tensile strength of at least about 550 grams per centimeter at 50% machine-direction elongation. The degree of bonding of the precursor web is specifically selected to facilitate handling of the web, with the contemplation that higher strength fabrics can be achieved if the filaments of the precursor web are maintained in a substantially continuous form. In accordance with the present invention, it is contemplated that the spunbond precursor web is subjected to bonding which provides no more than a minimum tensile strength which permits winding and unwinding of the precursor web. Thus, the minimal tensile strength of the precursor web is selected to facilitate efficient handling during manufacturing of the present low basis weight nonwoven fabric.
Other features and advantages of the present invention will become readily apparent from the following detailed description, the accompanying drawings, and the appended claims.